(1) Ca(II)-Calmodulin activated phosphoprotein phosphatase (calcineurin) was first isolated as an inhibitor of Ca(II)-calmodulin activated cyclic nucleotide phosphodiesterase from brain. It has since been demonstrated as a phosphoprotein phosphatase for the phosphorylated form of the heat stable inhibitor-1 from skeletal muscle. Since the enzyme is abundant in brain, the search for its natural substrate(s) was carried out with brain phosphoproteins. The results indicate that G-substrate, DARPP-32 and protein K-F are good substrates for the enzyme and they could indeed be the natural substrates of calcineurin in brain; (2) Attempt to demonstrate that phosphrylation is involved in regulating the aminoacyl-tRNA synthetase as described in the literature was not successful. Our results indicate (i) the purified synthetase complex does not possess specific protein kinase for the phosphorylation of the synthetase, (ii) the phosphorylation induced the inactivation of synthetase activity reported in the literature is likely due to improper control of the concentration of Mg(II), sulfhydryl reagent and other stabilizing agents for the enzymes; (3) Study of actomyosin ATPase mechanism revealed that the reaction proceeded via a "six-state" kinetic model and the rate-limiting step in the catalytic cycle is not Pi release from acto-S-1 ADP Pi or the ATP hydrolysis step. It appears that the rate-limiting step in the cycle is a conformational change step involving myosin ADP Pi complex and the rate for this step is independent of actin. This rate limiting step may play an important role in determining the velocity of muscle contraction.